Last big chill suggests lower climate impact of carbon

Researchers use the conditions that prevailed during the peak of the last …

One of the key measures of the impact of atmospheric carbon dioxide is called the climate sensitivity, which provides an estimate of how much the planet will warm in response to a doubling of the CO2 concentration. This figure has been estimated using a variety of methods, producing a range of values; the Intergovernmental Panel on Climate Change estimates that the most likely value is 3 Kelvin, but recognizes there's a reasonable chance it could range anywhere from 2.4-4.5K. A new study that uses a climate model to evaluate the peak of the last glacial period, however, suggests that the IPCC's figure might be a bit high, and that very high values are overwhelmingly unlikely.

Glacial periods are triggered by small changes in the Earth's orbit. These aren't enough by themselves to alter the global climate, but they set off a drop in atmospheric CO2 and an expansion of ice, which reflects sunlight back to space. These feedbacks help the Earth enter a deep chill during glacial periods.

The study focuses on the peak of the last glacial cycle, called the Last Glacial Maximum, which took place around 20,000 years ago. The conditions the authors use include larger ice sheets, lower greenhouse gas concentrations, increased dust, and the changes in solar forcings driven by the orbital differences (a forcing is anything that can shift the climate). For the actual conditions, they obtained temperature data from pollen samples, ice cores, and ocean sediments. Combined, these samples cover about a quarter of the planet's surface, and provide one of the most detailed reconstructions of the temperature of the LGM.

So the authors have a set of inputs—the things that force the climate—and an estimate of the output, namely the global temperature. To estimate the climate sensitivity, they perform multiple runs of a suite of climate models, providing them different climate sensitivities, and perform a Bayesian analysis to figure out which values are consistent with the state of the planet. For this work, they used a set of 47 different versions of the University of Victoria climate model, which they consider "intermediate complexity." In this case, that means that it incorporates information on dust, but doesn't include feedback changes in clouds or winds.

To look at climate sensitivity, the authors short-circuited the actual role of carbon dioxide, and simply changed its impact by adjusting the amount of infrared radiation that escapes through the atmosphere (carbon dioxide acts by trapping this radiation). Each of the 47 different models has a different value for this escaping radiation, and so models different levels of greenhouse gas impact.

This approach let them set a number of limits on the climate sensitivity. For example, model runs where it was too low keep the planet warmer than it was at the LGM. In other words, if the contribution of reduced CO2 levels is too small, the changes in the remaining forcings aren't enough to trigger a deep glaciation. In the same way, high climate sensitivities produce an extremely cold planet, far colder than the LGM. In fact, climate sensitivities above 6K trigger a global glaciation, or snowball Earth—something that has happened in the past, but not for over half a billion years. "Our model thus suggests that large climate sensitivities cannot be reconciled with paleoclimatic and geologic evidence, and hence should be assigned near-zero probability," they conclude.

Overall, their best fitting model involved a climate sensitivity of 2.4K, a touch under the IPCC's best fit, and a range of likely values that's also generally lower than the IPCC's. That best fit, however, has some problems: the model has Antarctica 4K warmer than it actually was, but suggests the West Antarctic Ice Sheet was 7K cooler than the actual data indicates. It's not clear whether this is an issue with regional details, or an indication that the best global fit isn't actually a very good fit.

The authors offer three explanations for why their results provide a lower climate sensitivity than previous work: their temperature reconstruction suggests the LGM was warmer than earlier work; their temperature data comes from sources that are a bit more evenly distributed around the planet; and not all studies have included estimates of atmospheric dust.

This looks like a solid effort, and should be easy to extend to using other climate model ensembles, which would provide a greater degree of confidence in their climate sensitivity value (assuming that other models agree, of course). But, as the authors note, their results are sensitive to the estimates of global temperature, which in turn are dependent on where we've obtained information about the LGM. More widely dispersed sources of data from this time period would be the clearest way of improving all the estimates derived from this time.

The other thing worth noting is that there may be limits to how much a climate sensitivity from the past can tell us how the Earth will behave now. Because of different feedbacks and starting conditions, it's possible that the climate sensitivity at the height of a glacial era will be slightly different from that of an interglacial era like the one we're now in. Fortunately, whatever conditions that created a snowball Earth millions of years ago don't seem to currently apply.

297 Reader Comments

Compare this with the energy trapped by increased GHGs. Show us your math, or at least show us your source for the comparison. And given your history of being moderated for content-free trolling and abusive posts, you might want to consider something more substantial than pretending someone threatened you.

wouldn't want that now yes quite the history.

matter into new energy that is what has changed in the last one hundred years.

Compare this with the energy trapped by increased GHGs. Show us your math, or at least show us your source for the comparison. And given your history of being moderated for content-free trolling and abusive posts, you might want to consider something more substantial than pretending someone threatened you.

wouldn't want that now yes quite the history.

matter into new energy that is what has changed in the last one hundred years.

And it kicked into high gear around the 1960's??

also to the reader ignore these Ludites.do your own math.

Ironic since you've been ignoring every post in the thread with any math. Like my post which you conveniently ignored because it makes you look like a fool.

Compare this with the energy trapped by increased GHGs. Show us your math, or at least show us your source for the comparison. And given your history of being moderated for content-free trolling and abusive posts, you might want to consider something more substantial than pretending someone threatened you.

wouldn't want that now yes quite the history.

matter into new energy that is what has changed in the last one hundred years.

And it kicked into high gear around the 1960's??

also to the reader ignore these Ludites.do your own math.

Compare this with the energy trapped by increased GHGs.

prove it

If the gasses are absorbing all this new energy what would be the outcome?Increased motion?

Compare this with the energy trapped by increased GHGs. Show us your math, or at least show us your source for the comparison. And given your history of being moderated for content-free trolling and abusive posts, you might want to consider something more substantial than pretending someone threatened you.

wouldn't want that now yes quite the history.

matter into new energy that is what has changed in the last one hundred years.

And it kicked into high gear around the 1960's??

also to the reader ignore these Ludites.do your own math.

Compare this with the energy trapped by increased GHGs.

prove it

If the gasses are absorbing all this new energy what would be the outcome?Increased motion?

Hmm, you appear to be some variation on the Eliza-bot, with a focus on inane thermodynamic quips.

Compare this with the energy trapped by increased GHGs. Show us your math, or at least show us your source for the comparison. And given your history of being moderated for content-free trolling and abusive posts, you might want to consider something more substantial than pretending someone threatened you.

wouldn't want that now yes quite the history.

matter into new energy that is what has changed in the last one hundred years.

And it kicked into high gear around the 1960's??

also to the reader ignore these Ludites.do your own math.

Compare this with the energy trapped by increased GHGs.

prove it

If the gasses are absorbing all this new energy what would be the outcome?Increased motion?

Hmm, you appear to be some variation on the Eliza-bot, with a focus on inane thermodynamic quips.

How interesting, who programmed you?

I see insulting me is fair game but that is not counter trolling yeah.

Hmm, you appear to be some variation on the Eliza-bot, with a focus on inane thermodynamic quips.

How interesting, who programmed you?

I see insulting me is fair game but that is not counter trolling yeah.

I don't need you, to tell me what the book says, screw the book.

It's not counter-trolling because I actually engaged with what you said, demonstrated why you were wrong with links to the appropriate information and research.

You acknowledged none of that. You didn't recognize that the heat generated from manmade activities is known at the urban heat island, and accounted for in warming studies, you didn't acknowledge that your estimates of the impact of this heat is off by orders of magnitude in comparison to GHG contributions.

In short, you are spouting, not engaging with anyone. Therefore, you are either a troll, or an Eliza-bot. I thought I was giving you the benefit of the doubt by suggesting the latter.

Compare this with the energy trapped by increased GHGs. Show us your math, or at least show us your source for the comparison. And given your history of being moderated for content-free trolling and abusive posts, you might want to consider something more substantial than pretending someone threatened you.

wouldn't want that now yes quite the history.

matter into new energy that is what has changed in the last one hundred years.

And it kicked into high gear around the 1960's??

also to the reader ignore these Ludites.do your own math.

Compare this with the energy trapped by increased GHGs.

prove it

If the gasses are absorbing all this new energy what would be the outcome?Increased motion?

So what about the fact that sum total of all the energy you are talking about is 10,000 times less than what comes from the sun which is the energy everybody else is talking about being trapped by greenhouse gases?

So what about the fact that sum total of all the energy you are talking about is 10,000 times less than what comes from the sun which is the energy everybody else is talking about being trapped by greenhouse gases?

You need to think in a new context. There are organic levers and inorganic levers.Echo systems are primitive metabolisms.Without the lever there is no movement or motion. The lever is the basis of all technology.

You probably subscribe to space as being empty. That time is relative. The evolution is natural selection.

there are two ways of thinking of levers. class 1 class 2 class 3.So I know something not much but I get the connection.The problem is that the class 1 class 2 class 3 - is a poor encapsulation. No one can really even talk about it.

We define the lever as a Chase bound to three fulcrums. We define the mechanism as any given aggregate of levers that implement a lever.We define a machine as any given aggregate of mechanisms that implement a mechanism. We define Delta as a line that extends from the continuously inward to the continuously outward.

I think it is a terrible mistake to assume that infinity starts "here" in our universe.Infinity is bi-directional the outwardly infinite arises from the inwardly infinite.

Rather than time, it is size that is relative.

Stars are massive yet they function on the atomic levelBlack holes are profoundly massive yet they function on the subatomic level or a level so small we can't conceive of it.

As for disconnected trains of thought Yes that is called spatial reasoning You are all being logical.I don't do the logic as well as the spatial.So my discontented disconnected bits are a facet of my brain.

So to cry for moderator interference is well discriminatory.Racist even.

You may not get what I am saying but others have and do.

It is upsetting but then new ideas are upsetting aren't they.

Global warming has no place in the context of technology.

As a paying customer of this website I demand the all statistical constructs be banned.May we have a moderator please.

This is something AGW alarmists often bring up, but it's misleading FUD. While civilization developed around 10,000 years ago, "anatomically modern humans" have been around for about 200,000 years--even without the toolbox of civilization, we've lived through a variety of climate conditions and thrived as a species, and even more if you count "archaic homo sapiens" as essentially human and extend this time period back to 500,000 years ago. Localized climates have been both much warmer and much colder for long periods during that time, and while there's much evidence that colder climates limited human development there's little to no evidence that warmer climates have ever been a significant barrier given adequate water.

It's misleading to imply that the development of civilization had anything to do with the relative climate stability of the Holocene; in fact, most anthropologists link the beginnings of agriculture (and thus civilization) with the lowered carrying capacity of the Levant caused by the sudden cold of the Younger Dryas following an uncharacteristically warm period. Rather than being some sudden development made possible by climate stability, as AGW alarmists argue (in contradiction to most anthropologists, whose bailiwick this really is), the development of agriculture and civilization required several potentiating factors few of which are climate-related. Those that do involve climate include a localized period of climate stability long enough for groups like the Natufians to become semi-sedentary--but followed by climate instability which encourages these groups to begin planting crops to ensure a steady food supply instead of foraging. Indeed, a stable climate by itself does not lead to the development of agriculture and civilization but mitigates against it since a traditional hunter-gatherer lifestyle is supportable; a hunter-gatherer lifestyle likely dominated human existence for so long not because we lacked a long-term stable climate or any specific ability, but because the hunter-gatherer lifestyle was more than adequate to provide for relatively low human population densities (hunter-gatherers in most places at most times ate and lived well, and their remains show them to be taller and healthier and longer-lived than most agriculturalists were until very recently).

Anthropological evidence indicates that local climate instability following a warm stable period and/or insupportable population density increases are key in the development of agriculture. After that, the system can spread to regions of more stable climate and lower population densities as the population and trade influence of the agricultural culture grows and spreads. You can argue and I would agree that the relatively mild and stable climate conditions of the Holocene helped agricultural civilizations to spread, but it's the "mild" relative warmth compared to the earlier glacial period which is key rather than the "stable" part. Indeed, throughout the Holocene there have been many significant localized climate changes, shifting former breadbaskets into deserts and vice-versa.

Suffice it to say, the relative climate stability of the Holocene is not a precondition for the inception of civilization, which has much more to do with population densities and the happenstance of local flora combined with the right cultural toolkit, and any historical role it played in helping civilization to spread is irrelevant now that civilization is the dominant human paradigm. Humans have tolerated and thrived in a variety of climates for half a million years depending on where you want to draw your lines, and that was before we had the tools of advanced civilization to bend our environmental conditions to our will rather than just sit back and let come what may.

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These are conditions that mankind has literally never seen before.

Again, we've seen it all over nearly half a million years, or 200,000 if you want to be more discriminating, living through long periods of localized climate both far colder and far warmer than most of the world today's or the near future's. Advanced civilization gives us a greater adaptability than our ancestors, not a lessened adaptability. Your argument would only be true if you believe the opposite, that the toolkit of civilization makes us less able to survive and adapt than primitive hunter-gatherers. Is that what you really believe?

This is something AGW alarmists often bring up, but it's misleading FUD. While civilization developed around 10,000 years ago, "anatomically modern humans" have been around for about 200,000 years--even without the toolbox of civilization, we've lived through a variety of climate conditions and thrived as a species, and even more if you count "archaic homo sapiens" as essentially human and extend this time period back to 500,000 years ago. Localized climates have been both much warmer and much colder for long periods during that time, and while there's much evidence that colder climates limited human development there's little to no evidence that warmer climates have ever been a significant barrier given adequate water.

CO2 levels are higher now than any time during that extended period. Also, there are a lot more people depending on agriculture now, which could be far more sensitive to climatic shifts than more primitive hunter-gatherer societies.

Nobody doubts that humans will survive GW, so that's a bit of a strawman. How much pain our civilization will have to endure is an open question. And indeed, that 'adequate water' when you are talking about many billions of people is certainly not a given.

True, but it's only the resulting temperatures which are relevant to humans (ignoring for the time being ocean acidification), and during 500,000 years we've lived through climates much warmer and much colder than today's.

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Also, there are a lot more people depending on agriculture now, which could be far more sensitive to climatic shifts than more primitive hunter-gatherer societies.

This points to a problem with population sizes, though, not with projected temperatures in and of themselves. Indeed, the exploding population sizes of the 20th and 21st centuries are largely fueled by an unsustainable charity of the developed world, which gives away tens of millions of tons of food to the developing world each year (and has been for over 50 years) at no or little cost, and which pumps hundreds of billions of dollars of infrastructure aid into these regions on an ongoing basis. This results in unsustainable population growth, which for some reason we've decided is better than the economic colonialism of tying all aid to trade and strategic considerations. Developed nations produce a net surplus of food, and are projected to continue to do so for some time (even longer if we get our net immigration numbers under control).

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Nobody doubts that humans will survive GW, so that's a bit of a strawman. How much pain our civilization will have to endure is an open question. And indeed, that 'adequate water' when you are talking about many billions of people is certainly not a given.

I hope low to median U.N. population growth models are correct, otherwise we'll have much bigger problems than climate change to deal with much sooner. That's what this is really about. If the world's population were a billion we'd have so many resources per capita that none of the issues associated with AGW would ever manifest in threatening ways in the first place; if it were to plateau today at 7 billion, we'd still have only minor growing pains to account for before we'd be naturally switching to greener technologies anyway; at a plateau of 10+ billion, all clamoring for more developed lifestyles...well, that's a horse of a different color.

I make it clear that if a few hundred million people suffer here and there, if we lose some island and coastal cultures and some less developed interior cultures suffer due to climate change, that's a pity but it's a price I'm willing to pay and then some for preserving the most advanced and developed civilizations on the planet, and for allowing them to become more advanced and more developed. Most of the people who will most suffer from CO2 mitigation inaction are people who would not even exist if not for the developed world's ill-conceived food and economic aid over the last 50+ years, so it's morally a wash as far as I'm concerned. The civilizations which stand to have a net gain from CO2 mitigation inaction are the most developed and advanced; the rest are secondary concerns when looking at the big picture of human progress.

This is something AGW alarmists often bring up, but it's misleading FUD. While civilization developed around 10,000 years ago, "anatomically modern humans" have been around for about 200,000 years--even without the toolbox of civilization, we've lived through a variety of climate conditions and thrived as a species, and even more if you count "archaic homo sapiens" as essentially human and extend this time period back to 500,000 years ago.

Yes, all of that without civilization. I don't want to live in an uncivilized world, frankly. Do you think we can support 7-9 billion people living the way we used to when there were only a few hundred thousand to a few million of us? We can't even all live the way we do in the US today if everybody tried it, it would take four Earths. Someone else linked to the energy costs of ever-increased expansion and steady growth, and how those assumptions lead to us requiring the entire output of a sun in less than 2,000 years. We are not living sustainably today, even if climate change weren't an issue (and it is, a very significant issue according to the the weight of the evidence).

SergeiEsenin wrote:

Ozy wrote:

CO2 levels are higher now than any time during that extended period.

True, but it's only the resulting temperatures which are relevant to humans (ignoring for the time being ocean acidification), and during 500,000 years we've lived through climates much warmer and much colder than today's.

Not as a planet of farmers we haven't.

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Also, there are a lot more people depending on agriculture now, which could be far more sensitive to climatic shifts than more primitive hunter-gatherer societies.

This points to a problem with population sizes, though, not with projected temperatures in and of themselves.

Growing populations are a consequence of our advanced civilizations and so you can't ignore them in the equations when tallying up the costs of climate change.

I had a much longer post written up with more point-by-point rebuttals for both posts, but it boils down to this: Bottom line is that climate change threatens the security of our basic food resources (agriculture and fishing). This makes the food more expensive over time, not cheaper. This means more and more capital must be poured into producing enough food and getting it where it's needed, meaning less capital for investment in the magical CO2-scrubbing future technology you have faith we will come up with in a future unconstrained by mitigation measures (apparently not taking into consideration the cost of adaptation). Adaptation is more expensive than mitigation and carries a higher cost in human lives and productivity due to changes in climate affecting regions that had stable ones before. Look at how flooding in Thailand has brought global hard drive manufacture a halt and caused skyrocketing prices for hdds and thus new computers. Even within the continental US, flooding has a massive economic cost. Flooding for the affected regions in both cases is expected to get worse as time goes on. Adapting to this will be costly. Another expensive natural disaster is drought, which is also expected to worsen in the future. The general pattern is that the wet places will get wetter (not necessarily a good thing) and the dry places will get drier (arguably not a good thing at all). Both of these will strain our food production and manufacturing sectors, the fuels for our advanced civilization you think is so resilient. Will climate change bring us back to hunter-gatherer subsistence lifestyles? No. Will it hamper economic growth and advancement as we deal with the consequences? Yes. This is why mitigation is preferable to adaptation, as economists conclude. Your irresponsible techno-utopian ideals are flimsy at every level of scrutiny.

Some people are good at using scientific terminology in fairly grammatical constructions, and consequently fail to realize that they don't actually understand the terms they are using. (The best -- and most difficult -- teacher I ever had, made a point of not allowing such glibness to pass unchallenged.)

This superficial glibness is a far commoner phenomenon than is generally appreciated (but there are plenty of flacks who are willing to exploit the fact that people often don't really understand even the terminology, let alone the concepts, being used).

There are also people suffer from more extreme mental confusion, who may even appear to use language very precisely but are in fact unable to properly distinguish between logical thinking and mere confabulation. These people can sound incredibly intelligent, logical, and rational, but nonetheless tend to be found in mental institutions.

I think your comment contributes little to the substance of the discussion, whatever the opinions are. To the degree the evidence and the material is not addressed, and those making the claims are instead criticized in this personal manner, there can in fact be no useful discussion.

I think your comment contributes little to the substance of the discussion, whatever the opinions are. To the degree the evidence and the material is not addressed, and those making the claims are instead criticized in this personal manner, there can in fact be no useful discussion.

I think I get your point. But I think you also need to read my comment more carefully.

Have I mentioned lately how much I love Real Climate? Their own write-up on the paper and its limitations is pretty illuminating, but in the comments two of the authors shows up and discusses their criticisms. It's one of those cases where the comments can be even more instructive than the post itself. This is excellent stuff for people who want to know more about the work and its implications, I can't recommend it enough.